1. Field of the Invention
The invention relates to thermal viewers and the like wherein solid state detectors, e.g. diodes, are cooled to temperatures such as the boiling point of nitrogen (approx. 77.degree. K.), in order to detect thermal photons having wavelengths, in the range from 8-14 microns.
2. Description of the Prior Art
In the above viewers, the detectors are mounted in the vacuum between the inner and outer walls of a glass dewar which may, for example, be filled with liquid nitrogen. The detector is mounted on the inner wall which contacts the liquid nitrogen and faces the outer wall which is transparent to infrared or far infrared. While such viewers are dispensible in many field situations that arise in military actions, law enforcement and purely scientific endeavors; the need to supply liquid nitrogen severely limits the utility of these devices.
There has been considerable effort, therefore, to substitute some form of mechanical cooler in which the coolant is permanently sealed into the cooling system. Considerable success has been obtained with special designs of Stirling cycle and Villumeau coolers. The cooling function is directed toward the end of a cold finger that extends from the cooler. This finger extends into a well formed by the inner dewar wall and contacts the substantially flat circular portion of that wall at the bottom of the well which also contacts the detector inside the dewar. A rubber o-ring between the finger and a cylindrical portion of the well wall reduces convection losses from the interface.
An immediate problem is presented by the fact that the length of cold finger does not always equal the depth of the dewar well. It, therefore, has been necessary to make a portion of the cold finger out of elastic material. The material loses much of its elasticity at low temperature, but is still adequate to solve the problem thus far presented.
Another difficulty which has only been partially solved is the problem of mechanical vibrations in the cooler. Initially these vibrations were so severe that the presence of the cooler could be easily detected with relatively unsophisticated sound detection equipment even at fairly large distances. This has been greatly overcome by the use of balanced designs and more efficient cooling cycles. There still remains a small component of vibration which is transmitted to the dewar and detector. Aside from the stress problems this presents in these elements, which are already subjected to temperature stresses, this vibration also effects the quality of the image produced by the system. As will be seen the image resolution is degraded in an inverse relationshop to improvements that have been made in the detector.